There's a nap for that...  

And it's ten minutes long.

You heard me.

The 5-minute nap produced few benefits in comparison with the no-nap control. The 10-minute nap produced immediate improvements in all outcome measures (including sleep latency, subjective sleepiness, fatigue, vigor, and cognitive performance), with some of these benefits maintained for as long as 155 minutes. The 20-minute nap was associated with improvements emerging 35 minutes after napping and lasting up to 125 minutes after napping. The 30-minute nap produced a period of impaired alertness and performance immediately after napping, indicative of sleep inertia, followed by improvements lasting up to 155 minutes after the nap.

CONCLUSIONS:

These findings suggest that the 10-minute nap was overall the most effective afternoon nap duration of the nap lengths examined in this study. The implications from these results also suggest a need to consider a process occurring in the first 10 minutes of sleep that may account for the benefits associated with brief naps. [more]

Some obvious caveats:

1. The test group was miserably small (24) and non-representative.
2. The researchers were Australians

 My only problem is I seem to be on the road or in the field when I need a nap.

Google to the rescue (but not in my lifetime, I doubt).